In order to generate with precision a particular control frequency, systems have been developed which utilize divider-type integrated circuits based upon microprocessor technology and whose limiting frequencies for many applications may not be sufficiently high. Divider ICs use as a frequency standard quartz oscillators with a quartz frequency of 9 MHz.
While such frequencies suffice in many cases, they are not satisfactory in other situations in which a large number of intermediate frequencies must be generated from the quartz oscillator frequency. This is the case, for example, for very fine or precise control of speed-setting in synchronous motor operation.
For example, when the standard frequency of 9 MHz serves as a primary frequency and an integral divisor of 1500 is selected, a variation in the divisor by unity in the digital setting of the divisor can result in a control frequency change jump .DELTA.f of 4 HZ.
Such a frequency jump in the control of a synchronous motor can result in loss of synchronous characteristics to significant phase-current fluctuations.
The problem can be avoided by the use of a converter to shift the frequency range which is employed in the 100 MHz ultrahigh frequency range by, for example, increasing the frequency standard used as the primary frequency by a power of 10. This however, requires expensive microelectronic modules for frequency transformation.
It is also possible to provide fine-step control of a frequency or a finely selectable control frequency with the aid of a synthesizer. With a synthesizer, it is possible to mix two frequencies which can be obtained by an integral deviation of a frequency standard to generate respective subharmonics or submultiples of the frequency standard. The mixing may involve addition or subtraction of the two submultiple frequencies.
Synthesizers of conventional design utilizing these techniques are, however, relatively expensive and to use synthesizers of such cost to generate precisely controllable frequency outputs for the operations of synchronous motors is especially uneconomical because many of the characteristics of the high-quality synthesizer units thereby are unutilized.
It is also not possible with this system to drive a synchronous motor together with other motors of a drive cascade, (for example in a system for feeding a web material in paper manufacture, for driving pressure rolls, or for driving spinning machines) since it is not possible to generate special phase differences as may be required for such systems. For the simultaneous drive of the motors of such cascade, additional devices are required for phase control of the respective motors.